1. Field of the Invention
The preferred embodiments of the present invention are directed generally to rack-mounted server systems. More particularly, the preferred embodiments are directed to reducing in-rush current when powering-on a rack or multiple racks of servers at the same time.
2. Background of the Invention
As the state of computer technology advances, the size of computers, especially servers, continues to decrease. While early server systems may have had multiple servers (or computers) in a single equipment rack, modern server footprints have decreased dramatically. In fact, it is now possible to put ten or more modem servers in the same physical space that one early server previously occupied. Further, modem servers typically contain multiple microprocessors, in spite of their smaller footprint, so the power density of each server has substantially increased. The power density “ceiling” only a few years ago was 1–3 kilowatts of power in a 42U rack; however, modem 42U server equipment racks consume up to 20 kilowatts of power.
The Electronic Institute of America (EIA) has defined a standard for mounting electronic equipment that has been widely adopted among the industry standard server marketplace. The standard unit of measure for physical server height is a “U” where 1U is 1.750 inches. EIA racks commonly come in 19 inch widths, although there are some 23 inch widths as wells. Server heights (or widths, depending upon their orientation within the rack) are typically measured in integer multiples of the unit U. For example, a 2U server has a height of approximately three and a half inches. As of this writing, servers having a 1U height and having multiple processors are typical. Server equipment racks come in multiple heights, but 22U, 36U, 42U and 48U are the most common in data centers. It is not uncommon to have as many as forty-two servers (computers) in a single 42U tall rack. Each of the servers acts as an independent computer, and thus requires connection to a power source and draws power during operation.
Distribution of power to the servers in rack-mounted server systems of the related art takes place by means of a Power Distribution Unit (PDU). A PDU may be as simple as a location within the rack-mounted server system to plug a set of standard 120 or 220 volt AC connectors, together with a breaker, or may be as complex as a power and frequency conditioning system. Regardless of the complexity, PDUs of the related art are typically located inside the server equipment rack enclosure together with The rack mounted computers, or directly under the rack in the cable and air conditioning space underneath a raised floor. Due to the large number of power cords, network cables, keyboard cables, video cables, mouse cables, and the like, it is very common that the PDUs of the related art are at best inconveniently accessible for resetting of any internal breakers, and are extremely inaccessible in the event they need to be replaced. Stated otherwise, while the PDUs may represent some over-current protection for servers in the rack-mounted server system, they are not meant to be the primary element in over-current protection.
Related art servers typically have the ability to remember their previous operational state on loss of supply power, and return the server to the previous operational state upon return of supply power. For example, if a server system was already in a powered down condition at a loss of overall power, mechanisms within the server remember the previous state and do not attempt to automatically boot the server upon power being restored. If, however, the server was operating when power was lost, related art systems remember the previous operational condition, through the use of non-volatile memory, and attempt to restore the server to an operational state upon the return of power. This feature of returning to previous operational states upon return of power after a loss of power, in combination with server density (which is ever-increasing), produces in-rush current problems. Consider for purposes of explanation a rack-mounted server system having forty-two servers, all powered and operational. Further consider that power to the rack-mounted system is lost due to an unscheduled event (electric utility failure, electrician opening wrong breaker, or the like), thus causing all forty-two servers to shut down. Once power is restored, all forty-two servers, remembering their previous operational state, draw power and attempt to restart operations. In-rush currents associated with each of these servers simultaneously attempting to power-on, in the related art, can draw too much current for the power distribution system, including the PDUs. Thus, in the related art system, one or more of the PDUs may trip their breakers, or worse, the current in-rush may damage a PDU. In either case, the servers that receive power from the tripped or destroyed PDU are no longer operational. As was discussed above, while it is possible to reset the breakers on the PDUs, this is not an easy or efficient task as it requires a person to physically access the breaker to reset it. Moreover, replacing the PDUs may disable the rack-mounted server system for many hours or even days.
In-rush current can be described as a large current spike that is short in duration that occurs when power is first applied to a power supply. The current needed to charge bulk capacitors in the power supply and bulk capacitors on each computer system board appear to the power supply outputs like a short circuit for a very brief amount of time (typically less than 0.01 sec). During this short period of time, a large amount of current is drawn by the power supply to charge these capacitors causing a current surge at the power supply line cord input. Additionally, electric motors, for example fan motors, have start up current requirements that far exceed their steady state current draw. All of these current devices downstream of the power supply cause corresponding increase in current demand through the PDU's while the demands are being met.
PDUs and circuit breakers are typically cascaded in a power delivery system between the primary power input to the data center and each individual server, and must be compliant with various safety regulatory laws that vary from country to country. While laws may require certain safety margins to be followed when sizing circuit wiring and breakers in the equipment room, it is a very common mistake for users to over-load a PDU without actually knowing it. This is possible because most servers only draw a fraction of their fully rated power consumption when in “steady state” or idle conditions. Due to the random workloads imposed upon various servers, a user may add another computer to a PDU circuit that is already overloaded per specifications. This compounds the inrush current problem described above since most computer systems draw considerably more power during power up self test than during steady state operation (due to cache, memory and CPU diagnostic tests being run in parallel with hard disk drives being spun up).
Thus, what is needed in the art is a mechanism to allow server systems to use the beneficial feature of returning to their previous operational state upon return of power without the possibility of tripping or otherwise destroying the relatively inaccessible PDUs.